Motor control system for automatic machine tools



Dec. 15, 1959 F. w. CUNNINGHAM 2,916,931

MOTOR CONTROL SYSTEM FOR AUTOMATIC MACHINE TOOLS Original Filed Aug. 2.1951 I06 I05 I05 SENSITIVE RELAY "POSITION SIGNAL INVENTOR. FECDEElCK\nl. CUNNINGHAM ArrogA/zk MOTOR CONTROL SYSTEM OR AUTOMATIC MACHINETODLS Frederick W. Cunningham, Stamford, Conn., assignor to AmericanBosch Anna Corporation, a corporation of New York Original applicationAugust 2, 1951, Serial No. 240,018,

now Patent No. 2,741,732, dated April 10, 1956. Divided and thisapplication February 28, 1956, Serial No. 568,288

Uite

the operation of the machine tool is controlled by a,

voltage proportional in magnitude to the desired displacement of thetool from the defined Zero position. In the usual machine tools.carriage for the tool or work is driven through a drive screw by somemotive means, which for automation, is controlled according to the errorbetween the desired position and the present position of the carriage.The indicated present. position, when measured by the number of turnsmade by the drive screw, may not be the actual position, however, due tobacklash between the drive screw and thecarriage.

For additional accuracy a loss-motion-compensator is installed tocompensate for backlashin the drive screw.

The backlash compensator includes a. measuring screw which is (hi on bythelead screw and is resiliently urged against the tool carriage. Thecarriage position is then measured by rotational displacement and thelongitudinal displacement of the measuring screw.

For a more complete understanding of the present invention reference maybe had to the accompanying diagram, in which The figure is a schematicview showing the loss-monon-compensator applied to a machine tool.

The machine tool control may be of the type fully described in US.Patent 2,741,732, and which essentially comprises a phase sensitiverelay responsive to the error between an electrical signal at terminals11, which is proportional to a desired displacement of the tool 20 orthe carriage 105 from a reference position, and a signal indicative ofthe present position of the carriage. The present position signal may beobtained from the potentiometer 12 which is energized by a constantvoltage and whose movable member is driven by motor 42 which also drivesthe lead screw 21. The diflerence or error signal operates the relay toapply a voltage of the correct sign and chosen magnitude (as inputvoltage E, for example) to the motor 42 and thereby causes motor 42 todrive the carriage 105 toward the desired position. When the errorsignal is zero, the lead screw should have driven the carriage to theposition indicated by the output of potentiometer 12. Usually, however,there is backlash between the drive screw 21 and the carriage 105 whichcauses a difference in the actual position of the carriage and itsindicated position.

For more accurate results in positioning the tool 20, the machine toolis equipped with the lost-motion-compensator which will be describedwith reference to an es Patent automatic lathe, such as shown in thedrawing, and includes the pickofi device 110 and the measuring screw106.

The lost-motion-compensator is a means of electrically compensating forthe backlash between the lead screw 21 and the tool 20.

The tool 20 is driven by means of the lead screw 21 which is threaded inthe carriage and is driven by output shaft 45 of motor 42. Gearing 45causes lead screw 21 to drive a similar metering screw 106 which is alsothreaded in carriage 105 but is free to slide longitudinally within itsbearings 107 and is always urged to the right by spring 108. The lefthand end of metering screw 106 carries a ball and knuckle joint 114 towhich is connected the armature 109 of the linear pickoff device 110.

it is evident that with any appreciable backlash between the drive screw21 and carriage 105 the position of tool 20 cannot be accuratelydetermined by the angular displacement of screw 21. Also, it should benoted that with meteringscrew 106 always urged to the right by spring108, the metering screw 105 will experience longitudinal displacementduring the lost motion period of screw 21. Thus the position of tool 20is accurately determinable from the angular displacement plus thelongitudinal displacement of metering screw 106, and conversely theerror in the position of tool 20 as determined from angular displacementof screw 21 alone is equal to the longitudinal displacement of meteringscrew 106. The pickoff device 110 is preferably of the differentialtransformer type where the primary winding 111 is energized by 5, andthe secondary windings 112 and 113 are connected in series opposition sothat the magnitude of the voltage across the output leads 117 isproportional to the longitudinal displacement of the armature 10% fromthe defined zero position. Leads 117 are connected in series with thepotentiometer 12, terminals 11 and the terminals of relay 10 so that thevoltage to relay 10 is the difference between the input signal atterminals 11 and the sum of the outputs of potentiometer 12 and thelinear pickoff 110. Thus, when the relay 10 is deenergized, the positionof the tool 20 is proportional to the sum of the angular displacement ofscrew 21 plus the longitudinal displacement of screw 106.

Although this specification has described the automatic tool controlwith reference to an engine lathe, the invention is not to be restrictedto such use but may be applied with equal facility to other machinetools, such as milling machines and jigborers, for example.

I claim:

1. In a device of the character described, motive means, a driven memberoperatively connected to said motive means, a screw threaded in saiddriven member, means urging said screw axially, means operatively connecting said motive means and said screw for driving said screw, meansoperatively connected to said screw and responsive to linear motionthereof and means for effecting the energization of said motive means bysaid means operatively connected to said screw.

2. In a device of the character described, motive means, a drivenmember, operative connections between said motive means and said drivenmember, a precision screw in said member, spring means urging said screwaxially, means for driving said screw, linear means operativelyconnected to said screw responsive to linear motion thereof, and meansfor energizing said motive means by said means operatively connected tosaid screw.

3. In a backlash eliminator of the type described, driving means havingan output shaft, a driven member, operative connections between saidoutput shaft and said driven member for driving said member, controlmeans connected to said operative connections and adapted to controlsaid driving means to drive said output shaft to a predeterminedposition, said control means comprising a precision screw in said drivenmember, spring means operatively connected to said screw to urge saidscrew axially in said driven member, means for driving said screw insynchronism with said output shaft, and means actuated by the lineardisplacement of said screw and adapted to modify the operation of saidcontrol means and to modify the position of said output shaftaccordingly.

4. In a backlash eliminator of'the type described, driving means havingan output shaft, a driven member, operative connections between saidoutput shaft and said driven member for driving said member, controlmeans connected to said operative connections and adapted to i controlsaid driving means to drive said output shaft to a predeterminedposition, said control means comprising a precision screw in said drivenmember, spring means operativeiy connected to said screw to urge saidscrew axially in said driven member, means for driving said screw insynchronism with said output shaft, and means for energizing saiddriving means to correct the position of said driven member bycompensating for backlash between said driven member and said outputshaft.

5. In a backlash eliminator of the type described, driving means havingan output shaft, a driven member, operative connections between saidoutput shaft and said driven member for driving said member, controlmeans connected to said operative connections and adapted to controlsaid driving means to drive said output shaft to a predeterminedposition, said control means comprising a precision screw in said drivenmember, spring means operatively connected to said screw to urge saidscrew axially in said driven member, means for driving said screw insynchronism with said output shaft, and means for energizing saiddriving means to correct the position of said driven member bycompensating for backlash between said driven member and said outputshaft, said means comprising signal means operatively connected to saidscrew and actuated by linear displacement of said screw and adapted toeffect actuation of said driving means according to said displacement tothereby reduce said displacement.

6. In a backlash eliminator of the type described, driving means havingan output shaft, a driven member, operative connections between saidoutput shaft and said driven member for driving said member, controlmeans connected to said operative connections and adapted to controlsaid driving means to drive said output shaft to a predeterminedposition, said control means comprising a precision screw in said drivenmember, spring means operatively connected to said screw to urge saidscrew axially in said driven member, means for driving said screw insynchronisrn with said output shaft, means actuated by the lineardisplacement of said screw and adapted to modify the operation of saidcontrol means and to modify the position of said output shaft accordinly, said means comprising signal means operatively connected to saidscrew and actuated by linear displacement of said screw and adapted toeffect actuation of said driving means according to said displacement tothereby reduce said displacement.

7. In a backlash eliminator of the type described, driving means havingan output shaft, a driven member, operative connections between saidoutput shaft and said driven member for driving said member controlmeans connected to said operative connections and adapted to controlsaid driving means to drive said output shaft to a predeterminedposition, said control means comprising a precision screw in said drivenmember, spring means operatively connected to said screw to urge saidscrew axially in said driven member, means for driving said 'meanscomprising linear pickofi signal means operativcly connected to saidscrew and actuated by linear displacement of said screw and adapted toeffect actuation of said driving means according to said displacement tothereby reduce displacement.

References Cited in the file of this patent UNITED STATES PATENTS DeVlieg July 24, 1951 Sean June 5, 1956

